This invention relates generally to the decontamination of spills and landfills and more particularly to such decontamination in situ using radio frequency heating.
Hazardous waste materials have been improperly deposited in thousands of sites all over the United States and, indeed, all over the world. Uncontrolled landfills have been used as convenient, but inadequate, disposal sites for industrially generated wastes, while other sites have been contaminated by accidental spills of hazardous materials. There are many sites where hazardous materials were spilled prior to the discovery of the hazardous nature of the materials being handled. Many hazardous materials found at these sites are stable, do not undergo environmental degradation at reasonably fast rates, have high boiling points and are considered toxic at very low concentration levels, and bio-accumulate in various species of the food chain at concentrations higher than that found in the environment.
Complete reclamation or isolation of such sites is preferred, but the cost associated with site disturbance by available methods has been considered prohibitive. The treatment of contaminated soil from such sites in an incinerator has not been a practical solution for several reasons, including the high cost of excavation and incineration, shortage of incineration capacity, inadequate methods and capacity for ash-disposal in the incinerators, and the hazards and risks associated with site disturbance and transportation. The treatment of uncontrolled landfills and spills would benefit from an in situ process that eliminates or alleviates these disadvantages and risks. Radio frequency (RF) heating applied in accordance with the present invention offers a viable in situ method for treatment of contaminated sites.
The term RF refers to frequencies used in wireless communication and represents a wide frequency range from 45 Hz to 10 gigahertz (GHz). However, the frequencies of interest for in situ soil heating lie principally between 0.5 to 45 MHz. For dry soils, in this frequency range, dipolar molecules absorb electromagnetic (EM) energy which is converted to heat due to dipole rotation and molecular vibration. The absorption of EM energy and conversion to heat occurs throughout the volume of the material and is not dependent on the relatively slow process of thermal conduction. The amount of energy dissipated in the heated soil is proportional to the dielectric constant, the loss tangent, frequency and the square of the field strength of the applied electromagnetic energy. The penetration depth of the applied fields is inversely related to frequency and the conductivity of the soil. Thus, for any given soil, frequency may be selected to provide the required penetration depth. Penetration of EM energy of a few to more than 50 meters can typically be achieved.
In situ heating of earth formations by high-frequency displacement currents is well known, particularly in the production of petroleum products, such as shale oil. Alternatively, heating by conduction currents at relatively low frequencies is also possible, but such heating is limited to earth that remains conductive, generally requiring the presence of water and, hence, operating at relatively low temperatures below the boiling point of water or requiring maintenance of pressure. Conduction heating at very high temperatures for the immobilization of radioactive components in soil is shown in Brouns, et al. U.S. Pat. No. 4,376,598, where conductive material was shown added to the soil to assure conduction, and the soil was shown heated to vitrification at temperatures as high as 1500.degree. C. whereat radioactive contaminants are fused with the silicates in the soil to form a glass or similar product which, upon, cooling forms a stable mass.
In situ heating of earth formations with RF is shown in Bridges, et al. U.S. Pat. No. Re. 30,738 and Kasevich, et al. U.S. Pat. No. 4,140,179. The former discloses the use of RF from a tri-plate line buried in the earth to heat a block of earth formations uniformly by displacement currents, leading to dielectric heating. The latter discloses radiating RF energy into the earth.